It is known in the field that certain maleimides, which could also generally be classified as monomers in that they possess a structure of the 1,2-ethylenic type, will undergo free radical polymerization with certain other olefinic monomers. For example, N-phenylmaleimide undergoes a free radical polymerization reaction with styrene, an olefinic monomer widely used in plastics technology, to form a copolymer.
It has also been suggested that the attainment of improved structural stiffness and high thermal stability of certain polymers to which maleimides such as N-phenylmaleimides have been added is due to the inhibition of the rotation of the imide residue around the backbone of the macromolecule.
For example, N-phenylmaleimide used in acrylonitrile-butadiene-styrene systems has raised the heat distortion temperature of the resulting resin from 110.degree. C. to 125.degree. C. See also U.S. Pat. No. 3,676,404 for a disclosure of the use of N-phenyl maleimide with certain acrylic polymers.
U.S. Pat. No. 4,173,595 describes thermosetting compositions prepared from bismaleimide and N-vinyl pyrrolidone, which can be further modified by the addition of unsaturated polyester, that result in cured systems displaying enhanced mechanical properties. See also U.S. Pat. No. 4,376,206. However, a disadvantage of bismaleimide is that it is not as soluble in most ordinary olefinic monomers as is N-phenylmaleimide.
Copolymers of styrene and N-phenylmaleimide (NPM) are described in U.S. Pat. No. 4,374,951. Similar copolymers are discussed in U.S. Pat. No. 4,618,655, which discloses imidization of a copolymer of styrene and maleic anhydride. See also Japanese Patent Application No. 60-90960, which suggests a variety of co-monomers for N-phenylmaleimide and related compounds. Barrales-Rienda, Gonzalez de la Campa and Gonzalez Ramos, J. Macromol. Sci. -Chem., A11(2), pp. 267-286 (1977) recite several copolymerizations of N-phenylmaleimide and observe improved stiffness and thermal stability. Copolymerization of N-phenylmaleimide with styrene is also shown in U.S. Pat. Nos. 4,374,951 and 4,604,438.
In the presentation of our co-pending application Ser. No. 167,674, of which this is a continuation-in-part, the examiner cited U.S. Pat. No. 2,650,215 to Strain, and particularly Example IV thereof, in which a more or less conventional unsaturated polyester composition was reacted with N-ethyl maleimide. It should be noted that Example IV does not teach anything of its results and, in fact, is inconsistent with the main teaching of the patent. The reference is concerned with esters which are made from unsaturated alcohols--the entire body of the description is restricted to the use of such esters, and the polyesters used in Example IV are not made with such unsaturated alcohols, nor is the formulation of Example IV within the claims. No Barcol hardness or heat distortion data are given as for the other examples. N-phenylmaleimide is listed under Example I as having resulted in a shortened gelation time, but only in a mixture with diethylene glycol bis (allyl carbonate). There is no teaching whatever in the Strain patent with respect to electrical or heat resistant properties of unsaturated polyesters such as are employed in the present disclosure when copolymerized with N-phenylmaleimide. The unsaturated alcohols used for Strain's teaching are referred to in the three patents cited by Strain at column 4, lines 18-19.
Various synthesis routes or schemes have been used to make the N-phenylmaleimides which we employ in polyester formulations. See, for example, M. P. Cava et al, Organic Syntheses, Vol. V, p. 944, U.S. Pat. Nos. 2,444,536 and 2,467,835. U.S. Pat. No. 4,376,206 relates that bismaleimides can be used to make heat-resistant thermoset resins, and cites U.S. Pat. No. 4,130,564 for a method of making maleimides and bismaleimides. A specific class of N-phenylmaleimides is disclosed in the '206 patent. See also U.S. Pat. Nos. 3,960,887, 4,111,879, 4,171,302 and particularly 4,154,737.
Our invention does not depend on a particular way of making the N-phenylmaleimide component. Any convenient method of making it will suffice.
By way of illustration, the following preparation of N-phenylmaleimide is described:
To a solution of 202 g, of maleic anhydride in 600 ml of acetone is added with agitation a solution containing 186.26 g of aniline, 33 g of triethylamine and 200 ml of acetone. After adequate reaction time, 6 g of sodium acetate and 250 g of acetic anhydride are added and the mixture is brought to reflux for about 2.5 hours. 335 g of acetone is distilled from the mixture and approximately 1000 g of water is added to the remaining reaction contents. The reaction product is filtered, washed with water and dried to give 200 g of product (86.3% yield) m.p. 86.degree. C.